The invention relates to an inductive displacement transducer which comprises a measuring bridge for measuring an a-c current having an angular frequency W, and which includes in one half thereof a pair of series coupled adjustable resistors, and in the other half a pair of induction coils having substantially identical dimensions and electrical characteristics and a movable magnetic armature common to both coils. In the symmetrical, balanced position of the magnetic armature, each coil has the same inductance L; each branch of the circuit including a coils also has the same ohmic resistance R.sub.s. In inductive displacement transducers of this type, the movement of the magnetic armature with respect to the induction coils is converted into a voltage which is proportional to the armature displacement, which voltage is measured in the diagonal of the bridge circuit. The pair of adjustable resistors serve to enable balancing of the bridge when the position of the magnetic armature is symmetrical relative to both coils.
Inductive displacement transducers are described in Handbook of Electrical Measurement of Mechanical Quantities by C. Rohrbach, VDI-Verlag, Duesseldorf, 1967. Section 4.4 of this reference discloses a transversal-armature transducer which consists of a bridge branch having a pair of identical coaxial coils, each of which is disposed about a leg of a pair of oppositely disposed, U-shaped magnetic cores. The magnetic armature of the transducer is disposed in the air gap between these magnetic cores. Section 4.5 of the reference discloses several different plunger-type armature transducers which include in at least one half of the bridge circuit a pair of either linear or curved coils containing an iron core magnetic armature which is movable either axially or pivotably.
In these known types of inductive displacement transducers, the adjustable resistors are generally shunted by a variable capacitor so that the displacement of the magnetic armature can be additionally determined relative to a position which is not symmetrical to both induction coils. To do this, it is necessary to first balance the phase shift between the applied voltage and the a-c voltage present in the bridge diagonal by means of the capacitor, and then balance the bridge by means of the pair of adjustable resistors. The determination of the linear displacement of the magnetic armature is, thus, quite complicated, since three electrical components must first be adjusted, and then the a-c voltage, which appears in the bridge diagonal and is proportional to the displacement of the magnetic armature, must be measured. Moreover, a phase difference usually exists between the applied a-c voltage and that present in the bridge diagonal in such transducers, this difference being a function of the position of the magnetic armature. In order to achieve accurate measurements, this phase difference must be balanced.